Liquid rocket engines operate at high pressure in order to force the fuel and oxidizer into the combustion chamber. To supply such a pressure to the fuel and the oxidizer, some rocket engines use fuel and oxidizer tanks pressurized to a pressure sufficient to force propellant into the rocket combustion chamber. In such engines, however, the pressurization system and tank mass significantly adds to the mass of the rocket system. Certain rocket engines do not use a pressure-fed system and instead use one or more pumps to supply fuel and oxidizer to the combustion chamber. Such pump-fed engines require a source of energy to drive the pump or pumps.
Several methods are known for driving one or more pumps for pressurization of fuel and oxidizer. In one method that is used in a gas generator type of system, a portion of the propellant at the output of the pump, or an auxiliary monopropellant, is burned to produce hot gas. The hot gas drives a turbine or a gas motor to provide mechanical power for the pump. The resulting exhaust from that motor is then ejected overboard. In these systems, some loss of performance occurs as that propellant that is burned in the gas generator is not available to produce thrust. In another known method called an expander cycle, which is used in liquid oxygen/hydrogen engines, the fuel that comes out of the pump is used to cool the rocket engine. The fuel is thereby heated and turns into gas, which is then used to drive the motor to provide power to the pump. The pump is operated at such a high pressure that the pressure of the output exhaust from the motor is high enough to be injected into the rocket engine and burned as fuel. The expander cycle works well with liquid hydrogen, because liquid hydrogen has a comparatively small heat of vaporization and a boiling point of around 20 degrees Kelvin. Accordingly, liquid hydrogen can absorb a large amount of heat when in the gaseous state. Therefore, hydrogen can be significantly superheated even though all of it flows through the engine or engine walls as coolant.
Such an expander cycle, however, may not be possible for other propellant combinations. For example, when kerosene is used as a fuel and oxygen as an oxidizer, kerosene cannot be evaporated because doing so would raise its temperature to the point where it undergoes undesirable chemical changes, breaks down or leaves deposits in the engine. In such cases, even though the kerosene can be used as a coolant, it becomes impractical to use as a working fluid to transfer energy to the motor for purposes of driving the pump.
Therefore, there is a need for an efficient propulsion system and method that is capable of adapting to a variety of propellant combinations.